A conventional container used for the above-mentioned purposes has a pull-off type container lid, and almost all cans which contain soft drinks and which are widely sold on the market at present have the above-mentioned type of can lid. A small number of cans having a press-breakable can lid are also manufactured and sold. These container lids can all be opened by a simple means. In short, all kinds of container lids are opened by pulling them off or press-breaking them so as to allow them to fulfill the function thereof excellently.
However, many kinds of problems concerning these can lids have been posed by the users and other people in the society. The important ones of these problems may be itemized as follows.
(1) It is necessary that both a pull-off type can lid a press-breakable can lid be opened with a smallest possible force because a force which can be applied to a can lid by a user is limited. In fact, a can lid may not fulfill its duties if too great a force is required to open it. Furthermore, even if a can lid is opened with a great deal of force, the hand or fingers of the user may possibly be hurt and, therefore, such a kind of can lid is attended with danger. Needless to say, however, it should not occur that a can lid is readily opened under some internal or external conditions when no one intends it to be opened.
(2) It is necessary that a can lid be designed such that it does not cause any danger after it has been opened since a sharp edge of a can lid after it has been opened often hurt the user. This often occurs while and after a can containing food is opened with a can opener. Accordingly, it is a matter of course that consideration be given to the safety of a can lid for the protection of users.
Consideration must be given to the edge of an opened end of a can as well as the edge of the lid of the can, which has been pulled off from the can. In short, the best safety measure against a can lid is to furnish both a base portion thereof and a separable portion thereof which is unitarily formed with the base portion with a certain kind of safety means.
(3) It is necessary that a tear portion of a can lid is not separated from a can body after the can lid has been opened. The production of cans having such a can lid has been strongly demanded recently. In fact, a tear portion of a can lid, which is made separable from a can body is mostly thrown away and difficult to recover. A tear portion of a can lid which has been thrown away has actually caused various kinds of environmental troubles. There are childred who tread on or play with such a part of a can lid to be hurt, and domestic animals which eat such a part of a can lid mistaking it for food to die.
(4) It is necessary that a can lid is never explosively opened due to the internal pressure and that a can lid which requires consideration to be given to its internal pressure be furnished with a pressure removing means.
A can lid causes a danger not only when it has an undesirable construction thereof but also when it is opened in an undesirable manner as mentioned above.
The inventor of the present invention formerly filed a patent application (JP, A, 52-024430) covering "Tearing Can Lid and Method of Producing Same", which will be described as follows, for the purpose of providing a can lid of a high safety.
A can lid according to the above-mentioned invention consists of a structure as shown in FIG. 1 and is opened by a method as illustrated in FIG. 2.
In a can lid shown in FIG. 1, a material plate 1 has an annular, cross-sectionally U-shaped or rectangular groove 2 formed by a deep drawing, and is folded in three layers at the bottom portion 3 of the groove 2 so as to form a caulked folded portion 4. Thus, a can lid having a tear portion with a score 5 on the outer surface of the folded portion 4 is formed.
In order to open this can lid, a small hard piece 6 is inserted into the annular groove 2 and inclined to press the inner wall 7 of the groove 2. As a result, the can lid is broken along the score 5. A tear member 8 inside the annular groove 2 is then removed or partially separated from the can lid to open the same. Namely, such a can lid can be opened by utilizing the principles of the lever and fulcrum with a comparatively small force, i.e. by utilizing the small piece 6 as a lever and the upper edge 9 of the annular groove 2 as a fulcrum. Since the fingers of a person never touch the score 5 when he opens the can lid, so that this can lid can be safely opened.
The condition of a can after it has been opened will be as follows.
A tear portion 11 left on a can body 10 is pressed against the outer surface of an inner end portion 12 of the bead of that portion of the can lid which consists of a folded thin plate, and in a caulked state and thus, the safety of the tear portion 11 is extremely high.
The score 5 may not be circularly provided, or it may be interrupted at certain points, so that the tear member 8 is not separated from the can body 10 after the can lid has been opened.
The above can lid, which is advantageous in many respects, still has problems as follows.
One of the problems is that, since the tear member 8 has a sharp cut edge 13 after the can lid has been opened, a person who touches the cut edge 13 may possibly be hurt. This possibly occurs when a person carries a can while gripping the half-separated tear member 8 or when a person opens a tear member to a small extent and then pull the same with his fingers to allow the can lid to be opened satisfactorily.
Another problem is that, when such a can lid, which sufficiently stands an ordinary internal pressure, is used for a can to which a comparatively high internal pressure is applied, the tear member of the can may be opened explosively to a fully extent the moment the cutting line is locally opened with a hard piece 6. In a case where the cutting line is provided at 360.degree., the tear member may be blown off. Accordingly, a can lid of such a can is extremely dangerous.